Method of extrusion cooking an edible material

ABSTRACT

Improved short length extrusion cooking devices (10) are provided which can achieve product throughput and quality characteristics of conventional long-barrel extruders. The short length extruders (10) of the invention include a relatively short barrel (14) having an inlet (18) and an endmost extrusion die (20). An elongated, helically flighted axially, rotatable screw assembly (22) is positioned within the barrel (14) and is coupled to motive means (39, 39a) for rotation of the assembly (22) at a speed of at least about 500 rpm. The device (10) may include an internal, apertured flow-restricting device (60, 110) which defines a mid-barrel choke point for the material being processed. An alternate extruder (120) is configured without a mid-barrel restriction and is designed to operate at essentially atmospheric internal pressure throughout the majority of the length of barrel (122) with a significant pressure rise in the final head (134) adjacent the extrusion die. Preferably, the barrel (14, 122) has an internal bore of generally frustoconical configuration with an effective length to maximum diameter ratio (L/D) of at least about 6. Novel extrusion processes and products are also provided, using extremely short extrusion barrel retention times to give cooked extrudates having essentially no amino acid or vitamin nutrient losses, and/or dense, highly cooked, low moisture feeds.

RELATED APPLICATION

This is a continuation-in-part of Ser. No. 08/743,561, filed Nov. 4,1996, now abandoned, which is a continuation-in-part of application Ser.No. 08/685,893, filed Jul. 18, 1996, now U.S. Pat. No. 5,694,833.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with an improved extrusioncooking device and method wherein the extruder is of minimal length toreduce equipment and maintenance costs. More particularly, the inventionpertains to such a device wherein the internal bore of the extruderbarrel is of tapered, generally frustoconical configuration and theextruder screw is correspondingly tapered. In the production of expandedfeed products, the extruder preferably includes means presenting amaterial flow restriction intermediate the inlet and extrusion die.Where dense, fully cooked, low moisture sterilized feeds are desired,the extruder is operated without a mid-barrel flow restriction, and theextruder is operated to generate high pressure within the barrelimmediately adjacent the extruder die. Extrusion devices in accordancewith the invention are operated at high speed and can essentially matchthe throughputs and product qualities of much larger machines.

2. Description of the Prior Art

Extrusion cooking devices have long been used in the manufacture of awide variety of edible and other products such as human and animalfeeds. Generally speaking, these types of extruders include an elongatedbarrel together with one or more internal, helically flighted, axiallyrotatable extrusion screws therein. The outlet of the extruder barrel isequipped with an apertured extrusion die. In use, a material to beprocessed is passed into and through the extruder barrel and issubjected to increasing levels of temperature, pressure and shear. Asthe material emerges from the extruder die, it is fully cooked andshaped and may typically be subdivided using a rotating knife assembly.Conventional extruders of this type are shown in U.S. Pat. Nos.4,763,569, 4,118,164 and 3,117,006.

Most conventional modem-day extrusion cookers are made up of a series ofinterconnected tubular barrel heads or sections with the internalflighted screw(s) also being sectionalized and mounted on powered,rotatable shaft(s). In order to achieve the desired level of cook, ithas been thought necessary to provide relatively long barrels andassociated screws. Thus, many high-output pet food machines may havefive to eight barrel sections and have a length of from about 10 to 20times the screw diameter. As can be appreciated, such long extruders areexpensive and moreover present problems associated with properlysupporting the extrusion screw(s) within the barrel. However, priorattempts at using relatively short extruders have not met with success,and have been plagued with problems of insufficient cook and/orrelatively low yields.

In recent years, attempts have been made to use extrusion equipment inthe fabrication of pelleted feeds. Extrusion is desirable in thiscontext because extrusion conditions effectively sterilize the products.However, pellets produced by traditional extrusion methods are often toohard and do not dissolve readily in water. Such hard pellets may passthrough the stomach of monogastric animals with the pellets remaininglargely intact and non-digested. Another problem associated withextrusion-produced feeds is that nutrients such as amino acids andvitamins may be substantially degraded and heat-denatured duringprocessing. On the other hand, products produced using conventionalpellet mills, though having many desirable physical and nutritionalproperties, are insufficiently heat processed and cooked so that harmfulbacteria may remain in the pelleted products. In response to theseproblems, it has been suggested to employ a dual component apparatus inthe form of an extruder (sometimes referred to as an "expander") whichis coupled to a pellet mill. The starting materials are thus cooked inthe extruder section, and ultimately formed in the attached pellet mill.This dual component apparatus is relatively expensive however,particularly for the production of animal feeds.

There is accordingly a need in the art for improved, low-cost, shortlength extruder devices which are essentially equal with conventionallong-barrel extruders in terms of product throughput and quality. Inaddition, there is a need for an extruder apparatus which can producefeeds containing substantially non-degraded nutrients which are highlycooked and have desirable digestion properties similar to those oftraditional feeds produced using a pellet mill.

SUMMARY OF THE INVENTION

The present overcomes the problems outlined above, and provides a shortlength cooking extruder and method which yields superior products atcommercially viable throughputs using an extruder substantially shorterin length than those of conventional design. Broadly speaking, theextruder of the invention includes the usual tubular barrel having aninlet and an outlet and presenting an inner surface defining anelongated bore. The extruder also includes an elongated, helicallyflighted screw assembly within the bore, motive means for axiallyrotating the screw assembly, and an apertured extrusion die disposedacross the barrel outlet.

However, a number of important structural features are incorporated intothe extruders hereof in order to achieve the ends of the invention.Thus, the internal bore of the barrel is preferably of generallyfrustoconical configuration for at least about 50% of the length of thebarrel between the inlet to the extrusion die and presents a generallydecreasing cross-sectional area along the bore length; preferably, thebarrel bore is of tapered, frustoconical configuration for substantiallythe entirety of the barrel length between the inlet and outlet.Moreover, one preferred extruder embodiment includes structure defininga mid-barrel material flow restriction, which is preferably in the formof an apertured flow-restricting device; this generates a mid-barrel dieresulting in a choke region of material during operation of theextruder. The screw assembly and flow-restriction are cooperativelydesigned in this embodiment so that the material displacement perrevolution of the screw assembly adjacent the upstream margin of theflow-restriction is less than the material displacement per revolutionadjacent the downstream margin of the flow-restriction.

In another embodiment especially designed for the production of dense,highly cooked pelleted feeds, the extruder is designed without amid-barrel restriction, but is configured so as to generate highpressure conditions in the barrel immediately adjacent the extruder die.In this way, the product is very rapidly cooked and formed withoutsubstantial nutrient degradation. In addition, the dense productsproduced using this embodiment have very desirable water absorption anddigestion properties.

The extruder barrels of the invention normally have inner bore-definingsurfaces configured to present spaced, helical rib sections along thelength thereof, these ribs assist in mixing and cooking of the materialduring travel along the short length of the extruder barrels. Thiseffect is augmented by the relatively high rotational speeds of thecorresponding screw assemblies; in practice, the screw assemblies arerotated at a speed of at least about 500 rpm, more preferably at leastabout 550 rpm, and even more preferably at least about 600 rpm. The mostpreferred range of rpm is from about 600-1200.

The short length extruders of the invention have a length to maximumdiameter ratio (L/D ratio) of up to about 6, and more preferably fromabout 3-6. Thus, devices in accordance with the invention can beproduced at a significantly lower cost as compared with conventionalcooking extruders. Furthermore, maintenance and parts replacement costsare lessened.

The extruders and methods in accordance with the invention areparticularly suited for the preparation of feed products, especiallyanimal feed products. Such products may be of the expanded variety, suchas typical pet foods, or more dense pellet-type products. The startingmaterials for expanded or dense feeds usually include a high proportionof grain at a level of at least about 40% by weight (e.g., corn, wheat,soy, milo, oats), and may include fats and other incidental ingredients.Expanded products in accordance with the invention would typically havea final (i.e., after drying) density of from about 15-25 lb/ft³, whereasdenser pellet-type products would normally have a final density of fromabout 30-50 lb/ft³. Broadly, therefore, products of the invention wouldhave final densities in the range of from about 15-50 lb/ft³.

It has also been found that products produced in accordance with thepresent invention exhibit essentially no loss of amino acid and/orvitamin content, i.e., no more than about a 10% loss as compared withthe respective total amino acid and/or vitamin contents of the startingrecipes, and most preferably less than 5% loss. Stated differently, theextrudates of the invention should have at least about 90% of thestarting total amino acid and/or vitamin content present therein in asubstantially nutritionally active and undegraded form, and morepreferably at least about 95% thereof. Total amino acids are derivedfrom the amino acids present in the starting ingredients and by theinclusion of amino acid additives. Such additives would include lysine,valine, methionine, arginine, threonine, tryptophan, histadine,isoleucine, and phenylalamine, either as a free amino acid or asresidues in more complex additives such as di-, tri- and otherpolypeptides. The type of vitamins would be dictated by nutritionalrequirements, and would typically include indigenous vitamins and/orvitamin premixes containing a variety of vitamins including vitamin A.The ability to maintain amino acid and/or vitamin contents is a distinctadvantage over conventional processing, wherein amino acid and vitamindegradation during extrusion cooking can be considerable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view depicting a preferred short lengthextruder in accordance with the invention;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1 and depictingthe mid-barrel die assembly of the extruder;

FIG. 3 is a sectional view similar to FIG. 2 but illustrating analternative mid-barrel die design;

FIG. 4 is a sectional view similar to that of FIG. 1 and illustrating ashort length extruder in accordance with the invention especiallyadapted for the production of low moisture, highly cooked, high bulkdensity animal feed products;

FIG. 5 is a side view illustrating the external configuration of thepreferred short length extruders in accordance with the invention;

FIG. 6 is a bar graph with a best fit logarithmic curve applied to thedata of a series of water absorption/pellet dispersion tests whereinextruded prior art pig feed was tested for initial crush resistance andfor crush resistance at one minute intervals during immersion of thefeed in 58° F. water;

FIG. 7 is a bar graph similar to that of FIG. 6 but illustrating thesame type of water absorption/pellet dispersion crush resistance testdata for a pig feed produced in accordance with the present invention;

FIG. 8 is a bar graph similar to that of FIGS. 6-7 but illustrating thesame type of water absorption/pellet dispersion crush resistance testdata for a prior art pig feed made using a pellet mill;

FIG. 9 is a scanning electron micrograph (SEM) illustrating thestructure of a conventional swine feed pellet prepared using a standardpellet mill; and

FIG. 10 is a scanning electron micrograph similar to that of FIG. 9 butillustrating the structure of a swine feed pellet in accordance with theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment of FIGS. 1-3

Turning now to the drawings, a short length extruder assembly 10designed for the production of expanded food products is illustrated inFIG. 1. Broadly speaking, the assembly 10 includes a preconditioner 12and an extruder 14. The latter includes an elongated tubular barrel 16having an inlet 18 and an endmost, apertured extrusion die 20. Anelongated, flighted, axially rotatable screw assembly 22 is disposedwithin barrel 16 along the length thereof.

In more detail, the preconditioner 12 is designed to initiallymoisturize and partially precook dry ingredients prior to passagethereof as a dough or the like into the inlet 18 of extruder 14. To thisend, the preconditioner 12 is typically in the form of an elongatedchamber equipped with rotatable internal paddles as well as injectionports for water and/or steam. A variety of preconditioners may be usedin the context of the invention. However, it is particularly preferredto use Wenger DDC preconditioners of the type described in U.S. Pat. No.4,752,139, incorporated by reference herein.

In the embodiment illustrated, the barrel 16 is made up of three axiallyaligned and interconnected tubular head sections, namely inlet head 24and second and third sections 26, 28. The inlet head 24 is configured topresent the upwardly opening extruder inlet 18 and is positioned beneaththe outlet of preconditioner 12 as shown. In addition, the inlet head 24has an apertured end wall 30 equipped with seals 32 for engaging sealblock 34. The screw assembly 22 is mounted on hexagonal drive shaft 36and is rotated via schematically depicted conventional bearing housing39 and electric motor 39a.

The second head 26 includes an outer metallic section 38 equipped withan external jacket 40. The latter has an inlet 42 and an outlet 44 so asto permit introduction of heating or cooling media (e.g., cold water orsteam) into the jacket, thus allowing indirect temperature control forthe head 26. In addition, the section 38 is provided with a pair ofthrough apertures 46, 48. As shown, an injection nipple 50 is locatedwithin aperture 46, whereas the aperture 48 has a removable plug 52therein.

The overall head 26 further includes a removable, stationary metallicsleeve 54 secured to the inner face of section 38. The sleeve 54 has aninternal surface 56 presenting helical rib sections 57 which defines anaxially extending bore 58. As shown, the thickness of sleeve 54increases along the length thereof such that the diameter of bore 58decreases between inlet head 24 and third head 28. The sleeve 54 alsohas transverse apertures 59 and 59a therethrough which are in alignmentwith barrel section apertures 46, 48 described previously. The end ofhead 26 remote from inlet head 24 is equipped with an apertured stator60 (see FIG. 2). The stator 60 includes an outboard flange 62 which issandwiched between the heads 26, 28 as shown, as well as an inwardlyextending annular segment. The segment 64 in turn has an innermostbearing ring 66 secured thereto by means of screws 68. In addition, thesegment 64 is provided with a series of six circumferentially spaced,circular holes 70 therethrough. FIG. 3 illustrates another rotor/statorassembly which is identical with that depicted in FIG. 2, save for thefact that, in lieu of the holes 70, a series of six circumferentiallyspaced slots 70a are provided.

Third head 28 is similar in many respects to head 26 and includes anouter tubular section 72 and an outboard jacket 74, the latter equippedwith an inlet 76 and outlet 78 for introduction of indirect cooling orheating media. Furthermore, the section 72 has transverse openings 80,82 therethrough which respectively receive nipple 84 and removable plug86.

A stationary, removable metallic sleeve 88 is positioned within section72 and has transverse apertures 89, 89a therethrough in registry withthe apertures 80, 82. The inner surface 90 of sleeve 88 presents helicalribs 89 and defines an axially extending central bore 92. The bore 92decreases in effective diameter between the end of barrel section 28adjacent section 26 and the end of the section 28 proximal to die 20.

The barrel 16 is completed by provision of a short annular spacer 94positioned adjacent the end of third barrel section 28 remote frombarrel section 26, together with endmost die 20. The latter in theembodiment shown is a simple metallic plate having a series of die holes96 therethrough.

The screw assembly 22 includes four rotatable elements mounted on theshaft 36 and interconnected in an end-to-end relationship. Inparticular, assembly 22 has an inlet screw section 98, a first screwsection 100, bearing rotor 102, and third screw section 104.

The second screw section 100 includes an elongated central shaft 106presenting an outer, generally frustoconical surface and outwardlyextending helical flighting 108. It is noteworthy that the pitch offlighting 108 is oriented at a pitch angle which is less than the pitchangle of the helical fighting 57 defined by surface 56 of sleeve 54.Moreover, it will be seen that the overall configuration of the screwsection 100 conforms with the decreasing diameter of bore 58, i.e., theouter periphery of the flighting 108 progressively decreases from theinlet end of the screw section 100 to the outlet end thereof adjacentrotor 102.

The rotor 102 is mounted on shaft 36 and includes an outermost, somewhatL-shaped in cross-section annular bearing 110 which is closely adjacentannular bearing segment 66 of stator 60. The rotor 102 and stator 60thus assists in stabilizing the screw assembly 22 during high speedrotation thereof.

The third screw section 104 is very similar to screw section 100. Thatis, the section 104 includes an elongated central shaft 112 presentingan outermost, frustoconical surface and helical flighting 114; thelatter is oriented at a pitch angle which is less than the pitch angleof the ribs 89.

Again referring to FIG. 1, it will be observed that the overall extruderbore defined by the sleeves 54 and 88 is of generally frustoconicalconfiguration leading from inlet 18 to die 20, i.e., the barrel borepresents a generally decreasing cross-sectional area along the lengththereof. Moreover, it will be seen that the effective length of theextruder from the remote end of inlet 18 to the end of barrel 16 (shownas dimension "L" in FIG. 1) versus the maximum diameter of the barrelbore (dimension "D" in FIG. 1) is relatively low, and preferably up toabout 6; the more preferred L/D ratio is from about 3-6. As used herein,"L/D ratio" refers to the ratio measured in accordance with theexemplary length and diameter illustrated in FIG. 1.

It will also be understood that the stator 60 and rotor 102cooperatively present a flow-restricting device intermediate the lengthof the barrel at the region of interconnection between barrel sections26 and 28. The overall flow-restricting device thus presents an upstreamface 1 16 and an opposed downstream face 118. The screw assembly 22 andthe flow-restricting device 60, 110 are cooperatively designed so thatthe material displacement per revolution of the assembly 22 adjacentface 116 is smaller than the material displacement per revolution of theassembly 22 adjacent the downstream face 118. Moreover, the assembly 22and device 60, 110 are designed so as to substantially continuouslymaintain the slots 70 forming a part of the flow-restricting device fullof material during operation of the extruder. In more detail, thematerial displacement per revolution of the screw assembly 22 adjacentdownstream face 118 is up to 40% greater than the material displacementof the screw adjacent the upstream face 118; more particularly, thedisplacement adjacent face 118 exceeds that adjacent face 116 by afactor of from about 15-40%. Also, the depressions between adjacent ribs89 in sleeve 88 are greater than the corresponding depressions in sleeve54. As a consequence, the free volume within the barrel bore downstreamof and adjacent flow-restricting device 60, 110 is greater than the freevolume adjacent and upstream of the flow-restricting device.Quantitatively speaking, the free volume within head 28 at the region offace 118 is up to about 30% greater than the free volume within head 26at the region of face 116, more preferably from 15-30% greater.

In typical operations employing extruders in accordance with theinvention to produce expanded feeds, an edible material to be processedis first formulated and then preconditioned, followed by passage intoand through the short length extruder. Normally, the startingingredients for the material to be processed include respectivequantities of protein and starch, along with amino acid and/or vitaminnutrient(s). Total amino acid content would include indigenous aminoacids as well as free amino acid additives as amino acids per se or aspolypeptides containing amino acid residues, and would range in contentup to about 5% by weight, and more preferably up to about 2% by weight.Total vitamin content would likewise be derived from that indigenouslypresent in the starting ingredients and as vitamin additives; totalvitamin content would range up to about 2% by weight. The proteincontent would normally be from about 12-50% by weight, more preferablyfrom about 18-32% by weight. Starch contents would range from about8-50% by weight, and more preferably from about 10-30% by weight. Asreadily understood by those skilled in the art, the protein and starchcontents are normally provided by the inclusion of desired protein- andstarch-bearing ingredients of animal or plant derivation. Commonstarch-bearing materials would be grains such as corn, wheat, milo,rice, beets, barley and mixtures thereof. Proteinaceous ingredientscould include soy, meat meal, and fish meal.

In the preferred preconditioner, the material is moisturized and atleast partially cooked. Preconditioning is normally carried out so thatthe product leaving the preconditioner has a total moisture content offrom about 15-40% by weight, and more preferably from about 22-28% byweight. The residence time in the preconditioner is usually from about15-150 seconds, and more preferably from about 90-150 seconds; and themaximum temperature in the preconditioner ranges from about 55-212° F.,and more preferably from about 180-200° F.

During passage through the extruder, the material is subjected toincreasing levels of temperature and shear and is normally fully cookedas it emerges from the extrusion die. Typical residence times of thematerial in the extruder barrel range from about 2-40 seconds, morepreferably from, about 2-15 seconds, still more preferably from about2-9 seconds, and most preferably from about 2-6 seconds. Maximumpressure levels achieved in the extruder barrel are normally from about150-1000 psi, and more preferably from about 300-500 psi. The maximumtemperature level achieved in the extruder barrel is from about 220-300°F., and more preferably from about 230-250° F.

During extrusion processing, the apertures of the flow-restrictingdevice 60, 110 are completely filled so as to create a choke in thebarrel at the zone of the flow-restricting device and a pressuredifferential across the device 60, 110 (i.e., the pressure is higher atface 116 as compared with the pressure of face 118). Moreover, owing tothe fact that the displacement per revolution of the screw assembly 22adjacent downstream face 118 is greater than that proximal to theupstream face 116, the free volume downstream of the flow-restrictiondevice is not fully choked with material. At a zone immediately adjacentthe die 20, another choke of material is formed in order to assuresmooth extrusion of the product through the die apertures.

Embodiment of FIGS. 4-5

FIG. 4 is a cross-sectional view of a short length extruder 120 similarin many respects to the extruder 14 of FIG. 1, but is especiallyconfigured for the manufacture of dense, highly cooked feed products.The extruder 120 is designed for use with the same type ofpreconditioner 12 described previously.

The extruder 120 includes an elongated tubular barrel 122 having aninlet 124 and an outlet 126, the latter being designed to receive anapertured die of conventional design (not shown). An elongated,flighted, axially rotatable screw assembly 128 is disposed within barrel122 along the length thereof.

The barrel 122 is made up of three axially aligned and interconnectedtubular head sections, namely inlet head 130 and second and thirdsections 132, 134. The inlet head 130 is configured to present theupwardly opening extruder inlet 124 and is positioned beneath the outletof a preconditioner such as preconditioner 12 (see FIG. 1). In addition,the inlet head 130 has an apertured end wall 136 equipped with seals 138for engaging seal block 140. The screw assembly 128 is mounted on ahexagonal drive shaft and is rotated via a conventional bearing housingand electric motor, in the manner of extruder 14.

The second head 132 includes an outer metallic section 142 equipped withan external jacket 144. The latter has an inlet 146 and an outlet 148 soas to permit introduction of heating or cooling media (e.g., cold wateror steam) into the jacket, thus allowing indirect temperature controlfor the head 132. In addition, the section 142 is provided with a pairof through apertures 150, 152. As shown, an injection nipple 154 islocated within aperture 150, whereas a second nipple 156 is positionedwithin aperture 152.

The overall head 132 further includes a removable, stationary metallicsleeve 158 secured to the inner face of section 142. The sleeve 158 hasan internal surface 160 presenting helical rib sections 162 whichdefines an axially extending bore 164. As shown, the thickness of sleeve158 increases along the length thereof such that the diameter of bore164 decreases between inlet head 130 and third head 134. The sleeve 158also has transverse apertures 166 and 168 therethrough which are inalignment with barrel section apertures 150, 152 described previously.

Third head 134 is similar in many respects to head 132 and includes anouter tubular section 170 and an outboard jacket 172, the latterequipped with an inlet 174 and outlet 176 for introduction of indirectcooling or heating media. Furthermore, the section 170 has transverseopenings 178, 180, 182 therethrough which respectively receive nipple184 and pressure gauges 186, 188.

A stationary, removable metallic sleeve 190 is positioned within section170 and has transverse apertures 192, 194, 196 therethrough in registrywith the apertures 178-182 respectively. The inner surface 198 of sleeve190 presents helical ribs 200 and defines an axially extending centralbore 202. The bore 202 decreases in effective diameter between the endof barrel section 134 adjacent section 132 and the end of the section134 proximal to the endmost extrusion die (not shown).

The barrel 122 is completed by provision of a die across the open facethereof. In many instances, a short annular spacer (not shown) may bepositioned adjacent the end of third barrel section 134 remote fromsecond barrel section 132, together with the endmost die.

The screw assembly 128 includes four rotatable elements mounted on thehexagonal drive shaft and interconnected in an end-to-end relationship.In particular, assembly 128 has a first inlet screw section 204, asecond screw section 206, flighted transition section 208, and thirdscrew section 210.

The second screw section 206 includes an elongated central shaft 212presenting an outer, generally frustoconical surface and outwardlyextending helical flighting 214. It is noteworthy that the pitch offlighting 214 is oriented at a pitch angle which is less than the pitchangle of the helical flighting 162 defined by surface 160 of sleeve 158.Moreover, it will be seen that the overall configuration of the screwsection 212 conforms with the decreasing diameter of bore 164, i.e., theouter periphery of the flighting 214 progressively decreases from theinlet end of the screw section 206 to the outlet end thereof adjacenttransition section 208.

The transition section 208 is in the form of a short cylindrical bodyhaving helical flighting 216 which is in alignment with helicalflighting 214 as shown.

The third screw section 210 is very similar to screw section 206. Thatis, the section 210 includes an elongated central shaft 218 presentingan outermost, frustoconical surface and helical flighting 220; thelatter is oriented at a pitch angle which is less than the pitch angleof the ribs 200. Moreover, the flighting 220 is aligned with flighting216 of transition section 208.

It will be observed that the overall extruder bore defined by thesleeves 158 and 190 is of generally frustoconical configuration leadingfrom inlet 124 to the endmost die, i.e., the barrel bore presents agenerally decreasing cross-sectional area along the length thereof. Theextruder 120 also has essentially the same L/D ratio as extruder 14described previously.

FIG. 5 illustrates an alternate external configuration for extruder 120.That is, the extruder 120a of FIG. 5 has the same internal configurationas extruder 120. However, the second and third heads 132a and 134a ofthe extruder barrel 122a are not equipped with external jackets. Rather,head cooling is effected by means of a series of radially outwardlyextending, circumferentially spaced cooling fins 222 and 224 provided onthe heads 132a, 134a, respectively. The FIG. 5 embodiment alsoillustrates a circular steam manifold pipe 226 disposed about the outletend of head 134a, with a total of four spaced apart separately valvedsteam injection pipe assemblies 228 coupled with manifold pipe 226. Eachof the assemblies 228 extends through the wall of barrel section 134a,so as to permit direct injection of steam into the confines of theextruder 120a. The manifold pipe 226 is covered by a perforate guard 230as shown.

The production of highly cooked, dense feed products using the apparatusof FIGS. 4-5 proceeds generally as described with reference to theproduction of expanded feed products, i.e., the starting formulation ispreconditioned and then fed into and through extruder 120 or 120a.However, in order to produce the desired feeds, some alteration of theprocess is necessary, most notably the moisture content of the startingmaterial and final product.

For example, a starting formulation would normally have a relativelyhigh grain content, at least about 60% by weight and more preferably atleast about 80% by weight. The grain fraction could be derived from anyof the aforementioned grain sources. Total protein for the startingformulations would usually range from about 12-50% by weight, morepreferably from about 18-32% by weight, whereas starch contents wouldrange from about 8-50% by weight and more preferably from about 10-30%by weight. Protein and/or starch can be provided by appropriate proteinand starch-bearing materials or through direct addition of desiredproteins and starches.

During preconditioning, the material is moisturized to a maximum ofabout 30% by weight, more commonly up to about 22% by weight.Temperature conditions within the preconditioner would range from about135-200° F. and more preferably from about 150-190° F. Residence timesin the preconditioner would generally be the same as those set forthabove for processing of expanded feed products.

During passage through the extruder, the preconditioned material is atleast partially cooked by the action of heat and shear. Residence timesof the preconditioned material in the extruder barrel are the same asthose described above, i.e., from about 2-40 seconds, more preferablyfrom about 2-15 seconds, still more preferably from about 2-9 secondsand most preferably from about 2-6 seconds. Maximum pressure conditionswithin the extruder barrel are experienced just upstream of the finalextrusion die, and generally range from about 25-400 psi, morepreferably from about 75-250 psi.

The dense product emerging from the extrusion die has a relatively lowmoisture of up to about 20% by weight, more preferably up to about 18%by weight. The hot extruder product can then be allowed to cool/dry inambient air to achieve final equilibrated moisture levels of from about10-15% by weight, more preferably around 12% by weight.

The following examples set forth preferred extrusion apparatus andmethods in accordance with the invention. It is to be understood thatthe invention is not so limited and nothing in the examples should betaken as a limitation upon the overall scope of the invention.

As used herein, "pellet durability index" and "PDI" refer to an artrecognized durability test described in Feed Manufacturing TechnologyIV, American Feed Association, Inc., 1994, pages 121-122 (and referencedinformation), incorporated by reference herein. In such a durabilitytest, the durability of pellets obtained immediately after cooling whenthe pellets have a temperature within 10° F. of ambient temperature.Durability is determined by tumbling a 500 g sample of pre-sievedpellets (to remove fines) for 5 minutes at 50 rpm in a dust-tight12"×12"×5" enclosure equipped with a 2"×9" internal plate affixedsymmetrically along a 9" side to a diagonal of one 12"×12" dimension ofthe enclosure. The enclosure is rotated about an axis perpendicular toand centered on the 12" sides thereof. After tumbling, fines are removedby screening, and the pellet sample is reweighed. Pellet durability isdefined as:

    durability=weight of pellets after tumbling/weight of pellets before tumbling×100

Example 1

In this example, a short length extruder in combination with apreconditioner was employed in the manufacture of high quality expandedpet food at commercial production rates.

The extruder was of the type depicted in FIG. 1, and consisted of threeheads. In particular, the extruder configuration used in Runs #1, #2 and#4 was made up of the following components (where all parts areidentified with Wenger Mfg. Co. part numbers): extruderbarrel--65695-001 (inlet head); 65676-001 (head No. 2); and 65689-001(head No. 3). Head No. 2 was equipped with internal liner 65691-001 anda stator 76598-001 between the second and third heads. Screwassembly--76597-001 (shaft); 65670-001 (inlet screw); 65671-001 (secondscrew section); 65906-003 (stationary shearlock between second and thirdscrew sections comprising 65907-001 (rotor) and 65909-001 (stator)); and65675-001 (third screw section). Final die--65534-009 (1" spacer);65421-001 (die plate); and 31350-779 (die insert giving 3/8" dieopenings). A rotating knife assembly was positioned adjacent the outletof the die for cutting the extrudate into a convenient size. The knifeassembly included the following: 19462-023 (knife holder) and ten knifeblades (19512-003). The extruder employed on Runs #3 and #5 wasidentical with that described above, except that the shaft employed wasWenger Part No. 76597-001 and the final screw section (Wenger Part No.65675-005) was of cut flight configuration.

The preconditioner used in both of these setups was a Wenger DDCpreconditioner having the standard 60--60 configuration.

In all of the five test runs, the starting pet food recipe was made upof 24% by weight poultry meal, 54% by weight corn meal, 8% by weightwheat, 8% by weight corn gluten meal, and 6% by weight soybean meal. Ineach case, the starting material was fed into and through thepreconditioner for moisturizing and partial cooking thereof, followed bypassage through the three head extruder. Water and sometimes steam wasinjected into the extruder barrel at the second and third head injectionports. Subsequent to extrusion, the product was conventionally dried toa moisture content of about 9-11% by weight.

The following table sets forth the operating conditions for thepreconditioner and extruder devices in the five runs.

                                      TABLE 1    __________________________________________________________________________                          RUN RUN RUN  RUN RUN                          #1  #2  #3   #4  #5    __________________________________________________________________________    RAW MATERIAL INFORMATION:    Dry Recipe Density                      kg/m.sup.3                          577 577 577  577 577    Dry Recipe Rate   kg/hr                          2000                              3000                                  3000 3500                                           3000    Feed Screw Speed  rpm  53  76  72   87  48    PRECONDITIONING INFORMATION    Preconditioner Speed                      rpm 125/250                              125/250                                  125/250                                       125/250                                           125/250    Steam Flow to Preconditioner                      kg/hr                          200 285 270  280 271    Water Flow to Preconditioner                      kg/hr                          300 540 540  655 482    Preconditioner Water                      ° C.       61  61    Temperature    EXTRUSION INFORMATION:    Extruder Shaft Speed                      rpm 592 592 592  592 592    Motor Load        %    63  60  83   88  63    Steam Flow to Extruder                      kg/hr                          --  --  --   --   60    Water Flow to Extruder                      kg/hr                           30  60  76   85  60    Control/Temperature-1st Head                      ° C.       83  87    Control/Temperature-2nd Head                      ° C.                           86 109 101  102 101    Control/Temperature-3rd Head                      ° C.                           93 110  76   98  98    Head/Pressure     kPa  3/NA                               3/NA                                    3/2068                                       2200                                           2250    FINAL PRODUCT INFORMATION:    Extruder Discharge Rate                      kg/hr                          320 400 320    Extruder Discharge Density                      kg/m.sup.3       368 352    Extruder Performance  Stable                              Stable                                  Stable                                       Stable                                           Stable    Duration of Run   min.                           15  15  8    15  30    Final Product Description                      in. 3/8 3/8 3/8  3/8 3/8                          chunk                              chunk                                  chunk                                       pellet                                           pellet    __________________________________________________________________________

All of the runs gave commercially acceptable, fully cooked and formedproducts. The bulk density of the product from Run #1 was found to beabout 19 lbs/ft³.

Example 2

In this example, a short length preconditioner/extruder of the typeshown in FIG. 4 was used to manufacture a high quality, dense, hard pigfinishing feed. The resultant product was equivalent if not superior tothose conventionally produced using an expander and pellet mill.

Specifically, the three-head extruder configuration used in Runs 6 and 7was made up of the following components (where all parts are identifiedwith Wenger Mfg. Co. part numbers): extruder barrel--65695-001 (inlethead); 65676-001 (head No. 2); and 65689-001 (head No. 3). Head No. 2was equipped with internal sleeve 65691-001, whereas head 3 also had aninternal sleeve, 76598-001. Screw assembly--76597-002 (shaft); 65670-001(inlet screw); 65671-001 (first screw section); 65906-001 (second screwsection) and 65676-001 (third screw section). Final die--66532-103 BH,65534-009 AD, 74010-953 NA, 74010-954 NA, with 13 inserts. A rotatingknife assembly was positioned adjacent the outlet of the die for cuttingthe extrudate into a convenient size. The knife assembly included thefollowing: 19462-001 (knife blade holder) and six knife blades(19430-007).

In the case of Runs 8 and 9, the extruder configuration was made up ofthe following components: extruder barrel--65695-001 (inlet head);65676-001 (head No. 2); and 65689-001 (head No. 3). Head No. 2 wasequipped with internal sleeve 65691-001, whereas head 3 also had aninternal sleeve, 76598-001. Screw assembly--76597-001 (shaft); 65670-001(inlet screw); 65671-001 (first screw section); 65658-015 (second screwsection); and 65675-001 (third screw section). Final die--6534-009 ADand 65421-001 BH. A rotating knife assembly was positioned adjacent theoutlet of the die for cutting the extrudate into a convenient size. Theknife assembly included the following: 19607-017 (knife blade holder)and five knife blades.

The preconditioner used in both of these setups was a Wenger Model 16DDC preconditioner having Configuration No. 377. The left and rightshafts were each equipped with a total of sixty beaters.

In Runs 6-9 inclusive, the starting recipe was made up of 76.96% byweight milo, 15.95% by weight soybean meal, 4.69% by weight tallow,0.94% by weight salt, 0.94% by weight calcium carbonate, 0.41% by weightvitamin premix, and 0.11% by weight lysine. In each case, the startingmaterial is fed into and through the preconditioner for moisturizing andpartial cooking thereof followed by passage through the three headextruder. Water was injected into the extruder barrel in Runs 7-9. Runs6 and 7 were somewhat unstable but Runs 8 and 9 were stable and gavegood, high density pig feeds. Subsequent to extrusion, the product wascooled using a multiple pass cooler to achieve final densities of 35lb/ft³ (Run 6), 36 lb/ft³ (Run 7), 45.4 lb/ft³ (Run 8), and 45.0 lb/ft³(Run 9).

The following table sets forth the operating conditions for thepreconditioner and extruder devices in the four runs.

                  TABLE 2    ______________________________________                   RUN   RUN     RUN     RUN                   #6    #7      #8      #9    ______________________________________    RAW MATERIAL    INFORMATION:    Dry Recipe Density                 kg/m.sup.3                         688     688   688   688    Dry Recipe Rate                 kg/hr   1500    1800  3000  4000    Feed Screw Speed                 rpm      31      37    64    78    PRECONDITIONING    INFORMATION    Steam Flow to                 kg/hr    62      54   210   283    Preconditioner    Water Flow to                 kg/hr   182      72    60    80    Preconditioner    Preconditioner                 kg/hr    75      36    0     0    Additive 1 Rate    Preconditioner                 ° C.                          69      73    85    86    Discharge Temp.    EXTRUSION    INFORMATION:    Extruder Shaft Speed                 rpm     592     592   592   591    Motor Load   %        70      95    47    38    Water Flow to Extruder                 kg/hr   --       36    30    40    Control/Temperature-                 ° C.                          66      58    56    49    2nd Head    Control/Temperature-                 ° C.                          90      98   106   117    3rd Head    Head/Pressure                 kPa     340     304   502    3/690    Knife Drive Speed                 rpm     350     350   610   770    FINAL PRODUCT    INFORMATION:    Extruder Discharge                 kg/m.sup.3                         548.7   560.9 675   673    Density    Final Product Description                         pig feed                                 pig feed                                       pig feed                                             pig feed    Run Rating           Fair    Fair  Good  Good    ______________________________________

The higher densities achieved in Runs 8 and 9 are believed chieflyattributable to the different die assembly employed as compared withRuns 6 and 7.

Although the extruder device specifically described herein is of thesingle screw type, it will be understood that short length twin screwextruders may also be fabricated and used in accordance with theinvention.

Example 3

In this example, swine feeds were produced in accordance with theinvention incorporating therein lysine and a vitamin premix containingvitamin A in order to determine the extent of lysine and vitamin Adegradation occurring during processing.

The three-head extruder used in these runs was of the type shown in FIG.4 and made up of the following components (where all parts areidentified with Wenger Mfg. Co. parts numbers): extruderbarrel--65695-001 (inlet head); 65676-001 (head No. 2); and 65689-001(head No. 3). Head No. 2 was equipped with internal sleeve 65691-001,whereas head 3 also had an internal sleeve, 76598-001. Screwassembly--76597-001 (shaft); 65670-001 (inlet screw); 65671-001 (firstscrew section); 65658-015 (second screw section); and 65675-001 (thirdscrew section). Final die--65534-009 AD, 65421-001 BH, 74010-955 NA,with ten inserts. A rotating knife assembly was positioned adjacent theoutlet of the die and included: 19607-017 (knife blade holder) and fiveknife blades. The preconditioner used in these runs was a Wenger Model16 DDC having Configuration No. 377. The left and right shafts were eachequipped with a total of 60 beaters.

In Runs 10-11, the starting recipe was made up of 76.96% by weight milo,15.95% by weight soybean meal, 4.69% by weight tallow, 0.94% by weightsalt, 0.94% by weight calcium carbonate, 0.41% by weight vitamin premix,and 0.11% by weight lysine. The following table sets forth the operatingconditions for the preconditioner and extruder device used in these tworuns.

                  TABLE 3    ______________________________________                         RUN #10                                RUN #11    ______________________________________    RAW MATERIAL INFORMATION:    Dry Recipe Moisture                       % wb    11.52    11.52    Dry Recipe Density kg/m.sup.3                               688      688    Dry Recipe Rate    kg/hr   3000     4000    Feed Screw Speed   rpm     64       78    PRECONDITIONING INFORMATION    Preconditioner Speed                       rpm     250      250    Steam Flow to Preconditioner                       kg/hr   210      283    Water Flow to Preconditioner                       kg/hr   60       80    Moisture Entering Extruder                       % wb    16.05    16.75    Preconditioner Discharge Temp.                       ° C.                               85       86    EXTRUSION INFORMATION:    Extruder Shaft Speed                       rpm     592      591    Motor Load         %       47       38    Water Flow to Extruder                       kg/hr   30       40    Control/Temperature-2nd Head                       ° C.                               56       49    Control/Temperature-3rd Head                       ° C.                               106      117    Head/Pressure      kPa     3/520    3/690    Knife Drive Speed  rpm     610      770    FINAL PRODUCT INFORMATION:    Extruder Discharge Moisture                       % wb    15.07    16.70    Extruder Discharge Density                       kg/m.sup.3                               657      673    Cooler Discharge Density                       lb/ft.sup.3                               45.4     45    Cooler Discharge Moisture                       % wb    13.5     11.98    Final Product Description  pig feed pig feed    Run Rating                 Good     Good    ______________________________________

Cooling of the respective extrudates was carried out in a two-passdryer/cooler. In the case of Run 10, the zone 1 temperature was 42° C.and zone 2 temperature was 39° C. Retention times were 2.7 min. past 1,and 5 min. past 2. Fan speeds 1-4 were 1597, 1638, 1078 and 1038 rpm,respectively. In Run 11, the zone 1 and zone 2 temperatures were 41° C.and 39° C. respectively, whereas retention times were 2.7 min. and 5min. respectively. Fan speeds 1-4 were 1579, 1635, 1078 and 1038 rpm,respectively.

The pig feed extrudates were analyzed and found to have for Run 10:piece density of 1.2245 g/ml, PDI (pellet durability index) of 99.4%,fat uptake of 8% by weight, and piece density after cooling of 1.2482g/ml. For Run 11: piece density of 1.203 g/ml, PDI of 99.0%, and fatuptake of 11.0% by weight.

In addition, the pig feed extrudates from Runs 10 and 11 were tested foravailable lysine, vitamin A and mold count. These test results are setforth below:

                  TABLE 4    ______________________________________              Available Lysine                            Vitamin A                                     Mold Count    Sample    (% by weight) (IU/kg)  (CFU/g)    ______________________________________    Raw recipe              0.70          1,777    300,000    Run 10    0.71          2,545    <10    Run 11    0.72          2,695    <10    ______________________________________

These data demonstrate that the products from Runs 10 and 11 experiencedno lysine or vitamin A loss, and complete destruction of molds,indicating that additional aflotoxins or other toxins will not be formedafter extrusion. Salmonella tests on the feeds were also negative. Theseresults are to be contrasted with typical available lysine and vitamin Alosses experienced in conventional extrusion processes. For example, pigfeeds produced using conventional equipment commonly give lysine lossesof 14-15% by weight, and vitamin A losses on the order of 40% by weight.

It is believed that the extremely short extruder residence timesachieved with the present invention give the essentially completeretention of lysine and vitamin content in the finished extrudates; theapproximate extruder barrel residence times for Runs 10-11 were measuredby color tracer injection and found to be about 3-4 seconds. At the sametime however, such extrudates are sufficiently cooked and otherwisehighly palatable products.

Example 4

In this series of tests, dense, relatively hard pig feed products wereproduced using an extruder as shown in FIG. 5. Two separate recipes wereused: in Runs #12-13, 80% by weight milo, 18% by weight soybean meal, 1%by weight calcium carbonate, and 1% by weight salt, with the dryingredients having a moisture content of 10.9% by weight, wet basis; inRuns #14-24, 80% by weight corn, 18% by weight soybean meal, 1% byweight calcium carbonate, and 1% by weight salt, with the dryingredients having a moisture content ranging from 9.39% (Run #22) to11.63% by weight, wet basis (Run #20). In all runs, the dry ingredientswere ground through a 1/16-inch screen, and during preconditioning, 2%by weight tallow was added.

The preconditioner used in all runs was a Wenger Model 16 DDC, usingconfiguration No. 377 where the left shaft was equipped with 60 beaters(12 at 75° forward, 24 at 90° neutral and 24 at -75° reverse), and theright shaft had 60 beaters (12 at 75° forward and 48 at -75° reverse).

In Runs #12-17 and 20-23, the extruder configuration included: extruderbarrel--65695-001 (inlet head 1), 65676-001 (head 2) and 65689-001 (head3); extruder sleeves--65691-001 (in head 2), and 76598-001 (in head 3);extruder shaft--76597-001; rotating elements mounted onshaft--65670-001, 65671-001, 65658-013 and 65675-001. For Runs #18-19,the extruder configuration included: extruder barrel--65695-001 (inlethead 1), 65676-001 (head 2), 65689-001 (head 3); extrudersleeves--65691-001 (in head 2), and 65693-001 (in head 3); extrudershaft--76597-001; rotating elements mounted on shaft--65670-001,65671-001, 65658-013 and 65675-001. The most preferred extruderconfiguration was used in Run #24 and was the same as that for Runs#12-17 and 20-23, except that the cone outlet screw had a 15° taper withan additional 1/4 spacer in front of the cone outlet screw to move itcloser to the discharge end of the extruder. This configuration isspecifically illustrated in FIG. 4.

The die and knife assembly used in Runs #12-21 included: dies andadaptors 53661-005 NA, 65421-001 BH and 74010-955 NA, with ten inserts,six 6 mm diameter holes for each insert, 15 mm land length; knifeholder--19462-023 carrying five 19430-003 knife blades. The assemblyused in Runs #22-24 included: dies and adaptors 53661-005 NA, 65421-001BH and 74010-752 NA, three 1/4" holes for each insert, 1/2" land length;knife holder--19462-023 carrying ten 19430-003 knife blades.

The following table sets forth the run conditions for this series ofexperiments.

                                      TABLE 5    __________________________________________________________________________                           Run                              Run                                 Run                                    Run                                       Run Run                                              Run                                                 Run                                                    Run                                                       Run                                                          Run                                                             Run                                                                Run                           #12                              #13                                 #14                                    #15                                       #16 #17                                              #18                                                 #19                                                    #20                                                       #21                                                          #22                                                             #23                                                                #24    __________________________________________________________________________    RAW MATERIAL INFORMATION:    Dry Recipe Density kg/m.sup.3                           620                              620                                 620                                    620                                       --  -- 620                                                 620                                                    620                                                       620                                                          620                                                             620                                                                620    Dry Recipe Rate    kg/hr                           3000                              4000                                 4000                                    5000                                       6000                                           4000                                              4000                                                 4000                                                    4000                                                       4000                                                          4000                                                             4000                                                                4000    Feed Screw Speed   rpm 63 93 92 123                                       136 91 89 93 88 83 83 85 83    PRECONDITIONING INFORMATION    Preconditioner Speed                       rpm 250                              250                                 250                                    250                                       250 250                                              250                                                 250                                                    250                                                       250                                                          250                                                             250                                                                250    Steam Flow to Preconditioner                       kg/hr                           210                              280                                 280                                    278                                       279 277                                              280                                                 280                                                    120                                                       120                                                          200                                                             200                                                                120    Water Flow to Preconditioner                       kg/hr                           60 80 120                                    150                                       240 280                                              120                                                 120                                                    40 40 160                                                             80 200    Moisture Entering Extruder                       % wb                           -- 17.17                                 19.33                                    18.99                                       18.68                                           20.76                                              18.18                                                 19.38                                                    17.02                                                       -- 17.85                                                             14.64                                                                --    Preconditioner Discharge Temp.                       ° C.                           87 83 83 75 72  80 84 87 60 -- 67 72 60    EXTRUSION INFORMATION:    Extruder Shaft Speed                       rpm 592                              592                                 592                                    592                                       592 522                                              650                                                 960                                                    960                                                       592                                                          592                                                             592                                                                592    Motor Load         %   60 73 46 55 59  53 65 88 86 73 47 69 70    Water Flow to Extruder                       kg/hr                           30 30 80 100                                       120 228                                              80 80 240                                                       240                                                          160                                                             40 200    Control/Temperature-2nd Head                       ° C.                           61 66 69 69 67  67 74 75 60 60 69 67 60    Control/Temperature-3rd Head                       ° C.                           122                              122                                 113                                    108                                       107 111                                              131                                                 142                                                    113                                                       113                                                          103                                                             125                                                                114    Head/Pressure      kPa -- -- -- -- --  -- 5350                                                 5670                                                    5590                                                       -- -- 1100                                                                780    Knife Drive Speed  rpm 545                              545                                 740                                    -- --  -- -- -- -- -- 1228                                                             1130                                                                540    FINAL PRODUCT INFORMATION:    Extruder Discharge Moisture                       % wb                           -- 15.03                                 17.28                                    18.79                                       18.30                                           24.13                                              16.49                                                 16.63                                                    15.59                                                       -- 18.30                                                             14.79                                                                --    Extruder Discharge Density                       kg/m.sup.3                           609                              617                                 640                                    625                                       617 600                                              620                                                 520                                                    592                                                       608                                                          640                                                             640                                                                649    Run Rating             Good                              Good                                 Good                                    Good                                       Good                                           Good                                              Good                                                 Fair                                                    Good                                                       -- Good                                                             Good                                                                Good    __________________________________________________________________________

Runs #12-17 were all run at an extruder shaft speed of less than 600rpm. In the case of Runs #18-20, the extruder shaft speed wassignificantly increased. This caused a very significant increase inpressure just upstream of the die and a corresponding significantincrease in extruder motor load. In the case of Run #23, a flightedtransition shearlock was provided between heads 2 and 3, which served tokeep the product from backing up in the middle of head 3 and resulted ineasier operational control. The final Run #24 employed a cone screw witha 15° taper, the flighted shearlock transition of Run #23, and with anadditional 1/4" spacer to move the end of the screw closer to thedischarge die. This gave the best product and performance of any of theruns.

All of the products were dense, relatively hard swine feed productshaving a high degree of cook, yet were capable of rapidly absorbingwater, making them ideal swine feed products.

The preferred dense animal feeds produced in accordance with the presentinvention are in the form of extruded bodies of low moisture (preferablyup to about 20% by weight moisture wet basis directly from the extruderand more preferably up to about 18% by weight) exhibiting at least about60% gelatinization (more preferably from about 65-85% gelatinization) ofthe starch-bearing components thereof, with a PDI of at least about 90and more preferably at least about 95. The products are thus highlycooked and have essentially no residual bacteria. The extruded bodiesare also relatively hard, and have bulk densities of at least about 28pounds per cubic foot and more preferably at least about 30 pounds percubic foot. Despite the hardness of the extruded bodies, they are alsoable to readily absorb moisture. Specifically, the products hereof, uponsubmersion in 58° F. water for a period of 4 minutes, should exhibit amaximum resistance to crushing which is less than about 70% (and morepreferably less than about 60%) of the maximum resistance to crushing ofthe product prior to water submersion. Furthermore, upon submersion in58° F. water for period of 8 minutes, the products of the inventionshould have a maximum resistance to crushing of up to about 40% (andmore preferably up to about 30%) of the maximum resistance to crushingof the product prior to water submersion. Such crush resistance testsare preferably performed using a Model TA.XT2 Texture Analyzer sold byTexture Technologies Corp of Scarsdale, N.Y.

In this connection, attention is directed to FIGS. 6-8. FIG. 6 is a bargraph with an applied best-fit logarithmic curve illustrating crushresistance tests using traditional extruded swine feeds. Note that afterfour minutes immersion in 58° F. water, the crush resistance of thetraditional extruded product was approximately 83.5% as compared withthe starting non-immersed product; and after eight minutes of immersion,the crushed resistance was about 78.6% on the same basis. FIG. 7 is asimilar graph and logarithmic curve showing the crush resistance ofswine feeds produced in accordance with the present invention. Theparticular product tested in this figure was from Run #13 of Example 4.As illustrated, after four minutes of immersion, the products of theinvention exhibited a crush resistance of about 52% as compared with thenon-immersed starting product, whereas after eight minutes, the crushresistance was only about 24.7%. FIG. 8 is similar to FIGS. 6 and 7, butdepicts the crush resistance properties of a conventional swine feedprepared by typical pelleting processes. The crush resistance data isvery similar to that of the present invention (52.7% crush resistanceafter four minutes immersion as compared with the starting product, and16.6% crush resistance after eight minutes immersion), thusdemonstrating that the extruded products of the present invention aresimilar to traditional pelleted products in terms of water absorptionand pellet dispersion.

This series of runs also demonstrated that the material undergoingextrusion experiences a very rapid increase in pressure just upstream ofthe final extrusion die. In fact, use of the dual pressure gauges 186and 188 (see FIG. 4) reveals that the pressure at the remote gauge 186is essentially atmospheric whereas the pressure at the adjacent gauge188 ranges from 780-1100 kPa (111-157 psi). Broadly speaking, thepressure within said extruder barrel at a point spaced rearwardly fromthe inner face of the extrusion die axially along the length of saidscrew assembly a distance equaling 1.5 times the largest diameter D ofthe extruder barrel should be essentially atmospheric. The pressurewithin said extruder barrel immediately adjacent said inner face of saidextrusion die should be at least about 100 psi, and more preferably atleast about 300 psi.

It has also been found that the "tip speed" of the extruder screwassembly can be an important parameter. The tip speed is the velocity ofthe extreme end of the extrusion screw closest the extrusion die. Thetip speed should be from about 400-1600 ft/min., more preferably fromabout 600-1200 ft/min., and most preferably about 700-900 ft/min.

In order to further illustrate the marked differences between pellets inaccordance with the invention and traditional products, comparativeswine feed pellets produced on a pellet mill and in accordance with thepresent invention were examined by taking scanning electron micrographsof the products. In each case, the representative pellet was slicedlongitudinally with a razor blade and standard SEM procedures werefollowed for obtaining the micrographs. The SEM of the conventionalpellet mill product is shown in FIG. 9, whereas the SEM of the improvedproduct of the invention is depicted in FIG. 10.

Referring first to FIG. 9, the illustrated round particles aresubstantially intact (i.e., not substantially gelatinized) starchparticles, with only a general flow pattern aligned with flow of pelletthrough the pellet mill die. In contrast, the FIG. 10 SEM demonstratesthat products in accordance with the invention having few if any intactstarch particles, with very pronounced flow pattern alignment. The FIG.10 SEM also illustrates a significant laminar structure which isbelieved to impart significant strength to the pellets.

In the preferred practice of the present invention, as the ingredientspass through the preconditioner, the protein and starch fractions aretransformed from a highly viscous, glassy state into or approaching arubbery dough. However, as the starting ingredient in this conditionenter the short length cooking extruder of the invention, thetemperature thereof rises to a point near to or even slightly above themelt transition temperature and the viscosity of the protein and starchfractions is reduced. As the materials exit through the final diehowever, the desired laminar structure is obtained and as thetemperature rapidly declines, the protein and starch fractions revert toa glassy state. At this point, the laminar structure is permanentlyretained in the final products. At the same time however, where denseproducts are desired conditions are controlled to limit any expansion ofthe product upon exiting the die. Generally, some degree of "die swell"is observed, but the overall expansion of the product upon extrusion issmall. The percentage of such expansion is measured as the diameter (orlargest cross-sectional dimension) of the product divided by thediameter (or largest cross-sectional dimension) of the die opening,times 100. The products of the invention typically have no more thanabout 30% expansion, more preferably up to about 20% expansion.

I claim:
 1. A method of extrusion cooking an edible material comprisingthe steps of:passing said edible material into the inlet of an elongatedextruder having a barrel equipped with an endmost extrusion die and aninternal, axially rotatable, flighted screw assembly within the barrel,said extruder having an L/D ratio of up to about 6; and rotating saidscrew assembly at a speed of at least about 500 rpm for advancing saidmaterial from said inlet along the length of said barrel and out saidextrusion die for at least partial cooking of the edible material, theresidence time of said material in said extruder barrel being from about2-9 seconds.
 2. The method of claim 1, said edible material including anutrient selected from the group consisting of an amino acid, a vitaminand mixtures thereof, at least about 90% of said nutrient in saidmaterial remaining in a substantially nutritionally active, undegradedform after passage thereof through said extrusion die.
 3. The method ofclaim 2, said extrudate having at least about 95% of said nutrienttherein in a substantially nutritionally active, undegraded form.
 4. Themethod of claim 2, said nutrient comprising lysine, valine, methionine,arginine, threonine, tryptophan, histadine, isoleucine, andphenylalamine.
 5. The method of claim 4, said nutrient comprising a freeamino acid.
 6. The method of claim 4, said nutrient comprising apolypeptide.
 7. The method of claim 2, said nutrient comprising vitaminA.
 8. A method of extrusion cooking an edible material comprising thesteps of:passing said edible material into the inlet of an elongatedextruder having a barrel, an internal, axially rotatable, flighted screwassembly within the barrel, and an endmost extrusion die having an innerface adjacent said screw assembly defining the outlet for the barrel;and rotating said screw assembly at a speed of at least 500 rpm foradvancing said material from said inlet along the length of said barreland out said extrusion die for at least partial cooking of the ediblematerial, the pressure within said extruder barrel at a point spacedrearwardly from the inner face of said extrusion die axially along thelength of said screw assembly a distance equaling 1.5 times the largestdiameter D of the extruder barrel being essentially atmospheric, and thepressure within said extruder barrel immediately adjacent said innerface of said extrusion die being at least about 100 psi.
 9. The methodof claim 8, said pressure within said extruder barrel immediatelyadjacent said inner face of said extrusion die being at least about 300psi.
 10. The method of claim 8, the tip speed of said screw assemblybeing from about 400-1600 ft/min.
 11. The method of claim 10, said tipspeed being from about 700-900 ft/min.
 12. A dense, hard, highly cookedanimal feed product comprising an extruded, edible body includingrespective quantities of protein and starch, said body having a bulkdensity of at least about 28 lb/ft³, the starch fraction of the bodybeing at least about 60% gelatinized, said product having a pelletdurability index of at least about 90, the product, upon submersion in58° F. water for a period of four minutes, having a maximum resistanceto crushing which is less than about 70% of the maximum resistance tocrushing of the product prior to said water submersion.
 13. The productof claim 12, said starch fraction being from about 65-85% gelatinized.14. The product of claim 12, said pellet durability index being at leastabout
 95. 15. The product of claim 12, the product, upon submersion in58° F. water for a period of eight minutes, having a maximum resistanceto crushing of up to about 40% of the maximum resistance to crushing onthe product prior to water submersion.
 16. The product of claim 12, saidproduct being a swine feed.
 17. The product of claim 12, said producthaving a moisture content of up to about 20% by weight immediately afterextrusion thereof.
 18. The product of claim 17, said moisture contentbeing up to about 18% by weight.
 19. The product of claim 12, saidproduct exhibiting no more than about 30% expansion.
 20. A dense, hard,highly cooked animal feed product comprising an extruded, edible bodyincluding respective quantities of protein and starch, said body havinga bulk density of at least about 28 lb/ft³, the starch fraction of thebody being at least about 60% gelatinized, said product having a pelletdurability index of at least about
 90. 21. A dense, hard, highly cookedanimal feed product comprising an extruded, edible body includingrespective quantities of protein and starch, said body having a bulkdensity of at least about 28 lb/ft³, the starch fraction of the productbeing at least about 60% gelatinized, the product, upon submersion in58° F. water for a period of four minutes, having a maximum resistanceto crushing which is less than about 70% of the maximum resistance tocrushing of the product prior to said water submersion.
 22. A method ofextrusion cooking an edible material comprising the steps of:passingsaid edible material into the inlet of an elongated extruder having abarrel equipped with an endmost extrusion die and an internal, axiallyrotatable, flighted screw assembly within the barrel; and rotating saidscrew assembly at a rotational speed of at least about 600 rpm foradvancing said material from said inlet along the length of said barreland out said extrusion die for at least partial cooking of the ediblematerial, the residence time of said material in said extruder barrelbeing from about 2-15 seconds.
 23. The method of claim 22, saidrotational speed being from about 600-1200 rpm.
 24. The method of claim22, said residence time being from about 2-6 seconds.